1. Field of the Invention
The invention relates to an ink composition advantageously employed in an ink jet recording method, a recording method and a recorded image utilizing the same, and an ink set and an ink head.
2. Description of the Related Art
The ink jet recording method is a method of recording an image by discharging and flying a liquid droplet of an ink composition (hereinafter also simply called “ink”) by dynamic energy or thermal energy, and depositing such liquid droplet on a recording material such as paper.
Usually, characteristics of an ink employed in the ink jet recording method (hereinafter also called “ink jet ink”) have been represented by physical properties such as a surface tension or a viscosity. It has been tried to 78 a drying property of the ink or to improve the quality of a formed image, by defining such physical properties.
For example, there is proposed an ink jet recording method by an ink droplet defined by a product of a Weber's number, including the surface tension as a parameter, and a Reynolds' number including the viscosity as a parameter. A high quality image is realized by maintaining the product of the Weber's number and the Reynolds' number within a certain range (cf. Japanese Examined Patent Publication JP-B2 2968010). Also there is proposed an ink jet recording method utilizing an ink with a surface tension within a specified range (cf. Japanese Examined Patent Publication JP-B2 63-65034 (1988)).
The surface tension used in the technologies described in the JP-B2 2968010 and JP-B2 63-65034 is a surface tension when a liquid surface reaches an equilibrium state, namely a static surface tension. Also the surface tension ordinarily employed as an index of the characteristics of the ink jet ink is a static surface tension (for example Akio Kinoshita et al., “Tokushu Kinou Inki no Jissai Gijutu (practical technologies of ink with special functions)”, 1st popular edition, CMC Co., Nov. 15, 1999, p. 6-7). In this manner, the characteristics of the ink jet ink are often indicated by the static surface tension among the surface tensions.
However, it is difficult to sufficiently represent the characteristics of the ink only by the static surface tension only.
In an ink jet recording apparatus for recording an image by the ink jet recording method, an ink is filled in an ink chamber of an ink head, and ink liquid droplets are discharged and made to fly in successive manner from a discharge port provided at a front end of the ink chamber and deposited on a recording material to record an image. At the continuous discharges of the ink, a newly generated surface of the ink, namely a new meniscus, is formed in the discharge port simultaneously with a discharge of an ink droplet, and the surface tension of the ink varies for example at the start of formation of the newly generated surface in the discharge port, at a state immediately before discharge, at a moment of discharge, during a flight of the droplet, at a moment of landing on the recording material, and during a penetration into the recording material such as paper. The surface tension of the ink changes from time to time between a surface tension in a state of slow motion such as at the start of formation of a newly generated surface at the discharge port or during the penetration into the recording material such as paper, and a state of fast motion such as at the moment of discharge. Herein, the state of the slow motion can be regarded as a static state and it may be considered that the surface tension changes from the static state to the dynamic state every second. Consequently, in order to sufficient represent the characteristics of the ink jet ink, it is necessary to consider a dynamic surface tension which is the surface tension while the liquid surface is in the course of reaching an equilibrium state.
The importance of the dynamic surface tension is described in various references. For example, Schwartz, J. evaluates static surface tension and dynamic surface tension of waterborne paints and indicates that the dynamic surface tension is an important fact in the formation of a coated film with the waterborne paint and a low dynamic surface tension was effect for forming an uniform excellent coated film (cf. “The Importance of Low Dynamic Surface Tension in Waterborne Coatings”, Journal of Coating Technology, US, Vol. 64, No. 812(1992.), pp65-74).
Also Medina, S. W. and Sutovich, M. N. discuss the importance of the dynamic surface tension in a high-speed printing, and suggest that the static surface tension, which is the surface tension when the liquid surface reaches an equilibrium, is effective as an index for representing the ink properties in a state of slow motion such as a penetration into paper and the like, but is not effective as an index for representing the ink properties particularly in a state of fast motion such as in a high-speed printing (cf. “Using Surfactants to Formulate VOC Comoliant Waterbased Inks”, American Ink Maker, United States, Vol. 72, No. 2(1994), pp32-38).
There are also proposed an ink composition, suitable for the ink jet recording method, with a defined relationship between the dynamic surface tension and the viscosity, and a recording method utilizing the same. This technology indicates that satisfactory printing characteristics can be obtained within a range where the ink composition satisfies a condition [dynamic surface tension (dyne/cm) at a life 0 msec]+[viscosity (cP)]=42 to 49 (cf. Japanese Examined Patent Publication JP-B2 2516218).
However, though the technology described in JP-B2 2516218 defines the dynamic surface tension by a conditional expression of adding the values of the dynamic surface tension and the viscosity, the value of such conditional expression is meaningless since the dynamic surface tension and the viscosity differ in the unit, i.e., differ in the dimension.
Also, this technology only defines the dynamic surface tension, but does not define the static surface tension. In the ink jet recording method, as explained in the foregoing, the surface tension of the ink has great difference between a state of slow motion which can be regarded as the static state and a state of fast motion which is the dynamic state, so that both the dynamic surface tension and the static surface tension are required for sufficiently representing the characteristics of the ink jet ink.
Further, this technology only defines the dynamic surface tension in a state of fast motion, particularly, of 0 msec, but does not define the dynamic surface tension in a state of slow motion. In the ink jet recording method, as explained in the foregoing, the surface tension of the ink varies from time to time between a surface tension in a state of slow motion and a surface tension in a state of fast motion, so that both the dynamic surface tension at a state of fast motion and the dynamic surface tension at a state of slow motion are required for sufficiently representing the characteristics of the ink jet ink.
Also this technology indicates that the surfactant does not contribute in reducing the dynamic surface tension. However the surfactant is known to generally contribute significantly to a wetting property of the ink jet ink on an internal wall of the ink chamber or a property of the ink on the recording material, and is not only an essential component in the ink but also an important factor for controlling the dynamic surface tension. For example, Schwartz, J., in the aforementioned “The Importance of Low Dynamic Surface Tension in Waterborne Coatings”, changing the static surface tension and the dynamic surface tension with certain surfactants and evaluating the influence thereof on the coated film formation, indicates that a reduction in the dynamic surface tension is effective in reducing a contraction of the coated film, a crater generation therein and a bubble entrapment therein. Also, Medina, S. W. and Sutovich, M. N. in the aforementioned “Using Surfactants to Formulate VOC Comoliant Waterbased Inks”, indicates that the dynamic surface tension is a barometer of ability by which the surfactant included in the ink gives way to a newly generated surface of the ink formed in sequence in the discharge port or to an interface between a droplet of the ink deposited in sequence on the recording material and the recording material. In other words, the surfactant is adsorbed on the newly generated surface of the ink or the interface between a droplet of the ink and the recording material and thereby lowers the surface tension of the ink and therefore the higher ability by which the surfactant gives way to a newly generated surface of the ink or the interface between a droplet of the ink and the recording material becomes, the higher effect of lowering the surface tension of the ink in the state of fast motion becomes and a dynamic surface tension of the ink is lowered. Nature of the surfactant exerts a great influence on the dynamic surface tension of the ink.